Tone detector with constant detection response time

ABSTRACT

A detector for detecting any one of the particular tone frequency signals in a range of tone frequency signals and which has a constant detection response time includes a converter operative in response to the particular tone frequency signals coupled thereto exceeding a predetermined threshold to develop square wave signals having substantially a 50% duty cycle. An integrator, coupled to the converter integrates the square wave signals to develop an integration voltage. The integration voltage reaches a predetermined amplitude level in response to the square wave signals in the constant detection response time causing actuation of a switching circuit which develops a detection signal.

United States Patent 1191 Brocker 11 3,792,290 [451 Feb. 12, 1974 TONE DETECTOR WITH CONSTANT DETECTION RESPONSE TIME John Race Brocker, Batavia, lll.

Primary Examiner.lohn S. Heyman Assistant Examiner-Ro E. Hart Attorney, Agent, or Firm-Marshall Dickler; Vince Rauner 57 ABSTRACT A detector for detecting any one of the particular tone 52 U.S. c1. 307/233, 328/133 frequency Signals in a range of tone qu y signals 51 Int. Cl. H03k 5/20 and which has a constant detection. response time [58] Field of Search 307/233, 155; 328/133, 134, cludes a converter operative in response to the partic- 3g3/140 ular tone frequency signals coupled thereto exceeding a'predetermined threshold to develop square wave sig- 56] References Ci d nals having substantially a 50% duty cycle. An integra- UNITED STATES PATENTS tor, coupled to the converter integrates the square I wave signals to develop an integration voltage. The integration voltage reaches a predetermined amplitude 3 6296] 1 l2/l97l Limberg 328/134 level in response to the square wave signals in the con- 3:652:134 3/1972 Hiscox Y 307/233 stant detection response time causing actuation of a 3,694,744 9/1972 Kalotav 307 233 Switching Circuit which develops a detection Signal- 3,735,257 5/1973 Roesner 307/233 3,636,270 1/1972 McIntosh 328 134 13 Clams, 1 Drawmg F'gme 10 ll /3 e /6 I R. E I. F. nun/0 c/ncu/r cowmrf AMPLIFIER W our/ ar I F 25 A 1 l8 7 UD! :2 L lM/TER sw C l9 INDICATOR Log/c A+ CIRCUIT BACKGROUND In radiocommunication systemsit is often necessary to transmit signalling tones in order to. contact particular units. In many systems, a number of tones, each of a different frequency, are sequentially transmitted, with each tone being transmitted for a short interval. Each. tone must be received and, detected within the particular interval. At the end: of a sequence, the unit may respond with an acknowledgement tone or sequence. If no acknowledgement is transmitted within a certaininterval, it is assumed the message was not received. This procedure may be carried on continuously in order to interrogate all units in the system.

In such systems,the time required to, interrogate each unit must be made as short aspossible, in order to maximizethe number of units which may be interrogated. Furthermore, each interrogation, and each tone detection for the tones in a sequence must be made as short as, possible in order to minimize equipment complexity and to minimize total system interrogation time. Each tone detection response time, and each interrogation time must then be made constant for any tone which may be transmitted and for any interrogation sequence.

The units in such systems are subjected tosubstantial temperature variations in normal usage. The tone detectorsin the units must have a constant tone detection response time over the temperature, range, in addition to. having a constant tone detection. time response for each tone which may be detected.

SUMMARY It is therefore an object of this invention to provide an improved tone detector having a constant tone detection response time.

i It is another object of this invention to provide an improved tone detector having a constant tone detection response time for any tone frequency signal which exceeds a predetermined amplitude in the range of tone frequency signals being employed.

Still another object of this invention is to provide a tone detector having a constant tone detection response time under substantialtemperature range variations.

In practicing this invention, a tone detector is provided which has a constant tone frequency detection time for detecting any one of theparticular tone frequency signals in a range of tone frequency signals. A phase splitter receives the particular tone frequency signals and develops first tone frequency signals and second tone frequency signals substantially 180 out of phase with the first tone frequency signals. The first and second tone frequency signals are coupled to a multivibrator which switches between first and second states in response thereto and develops a square wave signal train having substantially a 50 percent duty cyole. An integration circuit coupled to the multivibrator develops an integration voltage in response to the square wave signals, This integration voltage reaches. a predetermined amplitude level in a constant detection response time for any tone frequency signal. A switchingcircuit including a differential amplifier is coupled to the integration circuit. When the integration voltage reaches the predetermined amplitude level, the differential amplifier develops a detection signal.

THE DRAWING DETAILED DESCRIPTION Referring to the drawing, radio frequency (RF) sig nals modulated by particular tonefrequency signals are transmitted to antenna 10. Antenna 10 receives the RF signals and couples them to radio frequency circuit 11, which includes frequency selective circuits, and may or may not include amplifying circuits, The selected radio frequency signals are appliedto converter 12 which may include one or more stages of frequency conversion, to, provide an intermediate frequency (IF) signal. The intermediate frequency signal is amplified in stages indicated at 13, and limited in further stages indicated at 14. The limited intermediate frequency signal is applied to discriminator 15 which may beof known circuit configuration, and which is constructed to reproduce the audio or tone frequencymodulating signals. The output of the discriminator is applied to the audio output section 16, where it is amplified and applied to a loudspeaker 17, or other device for reproducing the modulation signals. I

The reproduced tone frequency signals developed at discriminator 15 are also coupled to. the detector circuit of this invention where they may be used to control an audio switch. 18, shown in dashed lines, that allows the audio to pass through the audio section 16 to speaker 17 thereby unsquelching; the receiver. Addi tionally, the detector circuit may be used to actuate an indicator 19 for indicating that a particular unit has been called, or to indicate which unit is calling.

The reproduced tone frequency signals developed at discriminator 15 are coupledto limiter 25 where the signals are amplified andlimited in order to control the amplitude of the desired signals and reduce any undesired extraneous noise signals which may be present. The amplified and limited tone frequency signals are coupled from limiter 25 to active filter 26. Active filter 26 may be an active filter such as commonly known in the art for passing particular tone frequency signals in a range, of tone frequency signals. It may be capable of passing only single particular tone frequency signals, or it may be variable, by the use of externally connected circuitry, not shown, in order to sequentially'pass a number of tone frequency signals in the range of tone frequency signals. In the preferred embodimennthe range of tone frequency signals extends from SOOHz to 2,0,00I-Iz. Active filter 26 is therefore designed to pass any particular tone frequency signal in this range, or any sequence of signals in the range.

If the tone frequency signals coupled to active tone filter 26 are the particular tone frequency signals to which active filter 26 is tuned, the signals will be coupled through active filter 26 to input terminal 29 of the detector. The particular tone frequency signals are coupled from input terminal 29 through coupling capacitor 30 to base electrode 32 of transistor 33. Emitter electrode 34 of transistor 33 is coupled to base electrode 35 of transistor 36, and collector electrode 37 of transistor 33 is connected to collector electrode 38 of transistor 36. Emitter electrode 39 of transistor 36 is coupled to one terminal of resistor 40. The other terminal of resistor 40 is coupled to ground potential. Collectorelectrodes 37 and 38 of transistors 33 and 36 are coupled to one terminal of resistor 42. The other terminal of resistor 42 is coupled to supply potential. Resistors 43 and 44 areserially connected between the supply potential and ground. The junction of resistors 33 and 34 is coupled to base electrode 32 of transistor 33 in order to supply bias potential for transistor 33.

Transistors 33 and 36 are coupled together in a Darlington configuration and act as aphase splitter for the tone frequency signals coupled to base electrode 32 of transistor 33. Two transistors, connected in a Darlington configuration, are necessary in order to provide the high input to low output impedance transformation and sufficient power gain required in the preferred embodiment.

The tone frequency signals coupled to base electrode 32 cause signals to be developed at emitter 39 of transistor 36, and also at collector electrodes 37 and 38 of transistors 33 and 36. The signals developed at collector electrodes 37 and 38 are 180 out of phase vyjjh thg signals developed at emitter electrode 39. The signals developed at collector electrodes 37 and 38 are coupled through capacitor 46 to junction 47, and the tone frequency signals developed at emitter electrode 39 are coupled through coupling capacitor 48 to junction 49.

Resistors 52 and 53 are serially connected between the source of potential and the ground potential. The junction of resistors 52 and 53 is coupled to the junction of resistors 54 and 55. One terminal of resistor 54 is coupled to junction 47, and one terminal of resistor 55 is coupled to junction 49. Resistors 52 and 53 form a voltage divider for providing a bias voltage to junctions 47 and 49. Resistors 54 and 55 act to isolate the signals at junctions 47 and 49. In the preferred embodiment the bias voltage at junctions 47 and 49 is approximately 0.15 volts. The purpose of this bias voltage will be more completely explained in a subsequent portion 'of this application.

Diode connected transistor 57 has its emitter electrode 58 coupled to junction 47 and its collector and base electrodes 59 coupled to base electrode 60 in transistor 61. Diode connected transistor 63 has its emitter electrode 64 coupled to junction 49, and its base and collector electrodes 65 coupled to base electrode 66 of transistor 67. Transistors 61 and 67 form a bistable multivibrator. The interconnection of transistors 61 and 67 in order toform a bistable multivibrator is well known in the art and will not be explained in detail.

Diode connected transistors 57 and 63 when conductive, will provide a voltage drop which offsets the base to emitter voltage drop of transistors 61 and 67, respectively. The voltage drop provided by transistors 57 and 63, and the biasing at junctions 47 and 49 provided by resistors 52 and 53 are such that a signal of negative polarity, and in excess of 0.15 volts at either junction 47 or 49 will cause one of transistors 61 and 67 to turn on. The bias voltage at junctions 47 and 49 provides a threshold then for operation of the bistable multivibrator. A negative input signal which, exceeds 0.15 volts in amplitude, at one of junctions 47 and 49 will cause the bistable multivibrator to switch states provided that the bistable is in the appropriate alternate state prior to the appearance of this signal. If the input signal does not exceed this threshold amplitude, bistable actuation will not occur.

The entire circuit shown in this specific embodiment is designed to be manufactured in integrated circuit form. All the transistors, therefore, are substantially identical. Transistors 57 and 63, in addition to providing a voltage drop which offsets the base to emitter voltage oftransistors 61 and 67 mathces the base emitter characteristics of transistors 61 and 67 in a manner in which compensates for the bias voltages at base electrodes 60 and 66 over the entire operating temperature range of this unit. As a consequence, transistors 61 and 67 will continue to operate in response to particular input signals at substantially the same point, 0.15 volts, over the entire operating temperature range of the units.

The negative half cycle of the tone frequency signals coupled to junction 47 will cause diode connected transistor 57 to forward bias when the voltage at junction 47 exceeds negative 0.15 volts and reduce the voltage at base electrode 60 of transistor 61. Transistor 61 will become reverse biased causing the voltage at collector electrode 70 to increase. The increased voltage at collector electrode 70 will be coupled to base electrode 66 of transistor 67 rendering transistor 67 conductive. With transistor 67 conductive, the voltage at collector electrode 71 will be reduced towards ground potential.

As the tone frequency signals developed at junctions 47 and 49 are out of phase, positive half cycles of the tone frequency signals will appear at junction 49 when negative half cycles appear at junction 47. The positive half cycles of the tone frequency signals developed at junction 49 will reverse bias diode connected transistors 63 so that the signal developed at junction 49 will not effect the switching of transistors 61 and 67 while the negative half cycles developed at junction 47 causes the change of state of transistor 61. When negative half cycles of the tone frequency signals are pres cut at the junction 49, and they exceed 0.15 volts, diode connected transistor 63 will be forward biased reducing the voltage developed at base electrode 66 of transistor 67. A reduction in the bias voltage at base electrode 66 will render transistor 67 non-conductive, causing an increase in the voltage developed at collector electrode 71. This increased voltage is coupled to base electrode 60 of transistor 61 rendering transistor 61 conductive, and reducing the voltage at collector electrode 70 towards ground potential. The positive half cycles of the tone frequency signals coupled to junction 47 reverse bias diode connected transistor 57 so that there will be no switching effect upon transistor 61 in the multivibrator when the negative half cycles at junction 49 cause the change of state of transistor 67.

As can be seen, then, successive half cycles of the tone frequency signals coupled to base electrode 32 of transistor 33 will cause the multivibrator consisting of transistors 61 and 67 to change states. A square wave signal train will therefore be developed at collector electrode 71 of transistor 67 having a frequency of substantially the particular tone frequency signals coupled to base electrode 32 of transistor 33. This square wave signal train will'vary substantially between supply and ground potential. The square wave signal train will be present as long as the particular tone frequency signals are coupled to base electrode 32 of transistor 33 and as long as they develop signals at junctions 47 and 49 in excess of 0.15 volts.

A square wave signal train has substantially a 50 percent duty cycle. That is, 50 percent of the time it is on or at substantially supply potential, and 50 percent of the time it is off or at substantially ground potential. The square wave signal train is coupled from collector electrode 71 of transistor 67 to an integrator consisting of resistor 73 and capacitor 74, where it is integrated to develop an integration voltage across capacitor 74. Integrating a square wave signal train having substantially a 50 percent duty cycle results in an integration voltage whose amplitude varies only in response to the length of time that the square wave signal is present. The integration voltage is fully independent of the freuqency of the square wave signal so long as the square wave signal retains a substantially 50 percent duty cycle. As a result, the integration voltage across capacitor 74 will reach a predetermined amplitude level in a predetermined period of time, for any particular tone frequency signal coupled to base electrode 32 of transistor 33 and resulting square wave signal train at collector electrode 71 of transistor 67. In the preferred embodiment, the predetermined amplitude level will be reached at milliseconds after the particular tone frequency signals are coupledto input terminal 29.

The integration voltage developedacross capacitor 74 is coupled to base electrode 76 of transistor 77. Transistor 77 and transistor 78 form a differential amplifier. Emitter electrodes 79 and 80 of transistors 77 and 78 are coupled together and to one terminal of re sistor 81. The other terminal of resistor 81 is coupled to ground potential. Series connected resistors 82 and 83 are coupled between supply and ground potential. The junction of resistors 82 and 83 is coupled to base electrode 85 of transistor 78, in order to provide a bias voltage to base electrode 85. Resistors 82 and 83 are selected such that the bias voltage at base electrode 85 is equal to the predetermined amplitude level of the voltage developed across capacitor 74 when a detection is to occur. I

When the amplitude of the integration voltage developed across capacitor 74 exceeds this predetermined amplitude level, transistor 77 will be rendered conductive developing a detection signal across resistor 86 coupled to collector electrode 87 of transistor 77. This detection signal will be coupled to base electrode 89 of the Darlington connected amplifier including transistors 90 and 91. The detection signal will be amplified by transistors 90 and 91 and coupled from collector electrodes 92 and 93 to output terminal 94 of the detector. The detection signal will be coupled from output terminal 94 to logic circuit 96 where it may be used to operate an indicator or audio switch such as indicator 19 and audio switch 18.

Logic circuit 96 may also include a number of other inputs from other detectors such as are shown by input leads 98. In certain embodiments, as noted above, a number of tones must be received in sequence in order to recognize a transmitted message. Logic circuit 96 will recognize each detection via signals coupled thereto from the detectors on input leads 98, and develop a reset signal in response to each detection. If the detector shown in the drawing has developed the detection signal, the reset signal will be coupled from logic circuit 96 to base electrode 97 of transistor 98 render ing transistor 98 conductive. With transistor 98 conductive, it will provide a short circuit path for the integration voltage developed across capacitor .74, discharging capacitor 74. This detector is now reset so as to accept a subsequently transmitted tone frequency signal.

Input leads 98 to logic circuit 96 may be coupled from the same detector or from a plurality of different detectors. At the end of the entire detection sequence, logic circuit 96 will develop the desired unit recognition signal.

In addition, logic circuit 96 may also be coupled to active filter 26 (coupling not shown) in order to vary the frequency of active filter 26 in response to each detection. This allows the same detector to be used for each tone frequency signal in the sequence of tone frequency signals received.

As mentioned above, transistor 61 and 67 form a bistable multivibrator. It is to be understood, however, that this invention is not limited to the use of a bistable multivibrator but may employ means for switching be tween a first and second state in response to input signals and developing a square wave signal having a 50 percent duty cycle. For example, a. resettable monostable multivibrator may be employed. If, however, a bistable multivibrator is employed as noted above, it can remain in either state when the tone frequency signals coupled to input terminal 29 terminate. Furthermore, when the unit is initially actuated, the bistable can be in either state. That is, transistor 67 at the termination of the signals coupled thereto can be conductive and transistor 61 can be non-conductive; or transistor 61 can be non-conductive and transistor 67 conductive. If transistor 67 remains non-conductive, the voltage developed at collector electrode 71 will cause a voltage to be maintained across integration capacitor 74. This will result in a detection signal always being present at output terminal 94 of the detector. In order to avoid such a condition from occurring, a reset circuit is employed in order to cause the multivibrator consisting of transistors 61 and 67 to switch to a second state wherein transistor 67 is conductive and the voltage at collector electrode 71 is reduced to zero; thus discharging capacitor 74.

An integration circuit consisting of resistor 100 and capacitor 101 is coupled to collector electrode of transistor 61. When transistor 61 is rendered conduc tive, (transistor 67 being non-conductive) collector electrode 70 approaches ground potential. This ground potential is coupled through resistor to one terminal of capacitor 101. The other terminal of capacitor 101 is coupled to supply potential. Capacitor 101 will begin to charge and develop a negative voltage at the junction of resistor 100 and capacitor 101. The value of capacitor 101 and resistor 100 are selected such that the voltage at the junction of capacitors 101 and resistor 100 will reach a predetermined level only after a predetermined period of time which is greater than the period of one-half cycle of the lowest tone frequency signals in the range of tone frequency signals which may be received by the detector. Should transistor 61 remain conductive, for this predetermined time period, the voltage developed at the junction of capacitor 101 and resistor 100 will reach this predetermined voltage level. The predetermined voltagelevel is coupled from the junction of capacitor 101 and resistor 100 to base electrode 103 of transistor 104 rendering transistor 104 conductive. When transistor 104 is rendered conductive, an increased positive voltage will be developed at collector electrode 105. This positive voltage will be coupled through resistor 106 to base electrode 108 of transistor 109 rendering transistor 109 conductive. With transistor 109 conductive, collector electrode 110 will approach ground potential which, in turn, causes collector electrode 71 of transistor 67 to approach ground potential. The ground potential at collector electrode 71 of transistor 67 will be coupled to base electrode 60 of transistor 61 rendering transistor 61 non-conductive. With transistor 61 non-conductive, collector electrode 70 will approach supply potential which is coupled to base electrode 66 of transistor 67 rendering transistor 67 conductive. The supply potential developed at collector electrode 70 of transistor 61 will also cause capacitor 101 to discharge. As capacitor 101 discharges, the voltage at base electrode 103 of transistor 104 is reduced rendering transistor 104 nonconductive. This, in turn, renders transistor 109 nonconductive, thus completing the reset cycle.

As can be seen, an improved tone detector has been provided for detecting particular tone frequency signals in a range of tone frequency signals and which has a constant detection response time. The detector has a constant detection response time for any tone frequency signal which exceeds a predetermined amplitude in the range of signals employed, and a constant tone detection response time under substantial temperature range variations.

I claim:

1. A detector for detecting any one of the particular sinusoidal tone frequency signals in a range of sinusoidal tone frequency signals and having a constant detection time period, said detector including in combination; converter means being operative in response to said particular sinusoidal tone frequency signals exceeding a predetermined threshold to develop square wave signals having substantially a 50 percent duty cycle, integration means coupled to said converter means and responsive to the square wave signals coupled thereto to develop an integration voltage, said integration voltage reaching a predetermined amplitude level in response to said square wave signals in said constant detection time period, and switch means coupled to said integration means and responsive to said integration voltage reaching said predetermined amplitude level to develop a detection signal.

2. The detector of claim 1 wherein said converter means includes, phase splitter means for receiving said particular sinusoidal tone freuqency signals, said phase splitter means being operative to develop first tone frequency signals, and second tone frequency signals substantially l80 out of phase with said first tone frequency signals, multivibrator means having first and second states coupled to said phase splitter means and responsive to said first and second tone frequency signals exceeding a predetermined amplitude to switch between said first and second states and develop said square wave signals.

3. A detector for detecting any one of the particular tone frequency signals in a range of signals and having 'a constant detection time period, said detector including in combination; first circuit means for receiving said particular tone frequency signals and having first and second outputs, said first circuit means being operative to develop said first tone frequency signals having a first phase at said first output and second tone frequency signals having a second phase at said second output, second circuit means having a first and second state coupled to said first and second output of said first circuit means, said second circuit means being responsive to said signals from said first and second outputs to switch between said first and second states and develop a square wave signal train having substantially a percent duty cycle, integration means coupled to said second circuit means and responsive to the square wave signals coupled thereto to develop an integration voltage, said integration voltage reaching a predetermined amplitude level in said constant detection time period, and switch means coupled to said integration means and responsive to said integration voltage reaching said predetermined amplitude level to develop a de tection signal.

4. The detector of claim 3 wherein said first circuit means includes phase splitter means said phase splitter means being operative to develop said first and second tone frequency signals, said second tone frequency signals being substantially out of phase with said first tone frequency signals, said first tone frequency signals having positive and negative half cycles-of substantially equal period, said second circuit means including third circuit means responsive to one of said positive and negative half cycles coupled thereto from said first output to switch to said first state, and fourth circuit means responsive to one of said positive and negative half cycles coupled thereto from said second output to switch to said second state.

5. The detector of claim 4 wherein said second circuit means includes bistable multivibrator means coupled to said phase splitter means first and second outputs, said bistable multivibrator means being responsive to said signals from said first output to switch to said first state and responsive to said signals from said second output to switch to said second state.

6. The detector of claim 5 wherein said second circuit means further includes bias means coupling said phase splitter means first and second outputs to said bistable multivibrator means, third and fourth circuit means said bias means biasing said bistable multivibrator means to switch states in response to one of said positive and negative half cycles of said particular tone frequency signals coupled thereto.

7. The detector of claim 6 wherein said second circuit means further includes reset circuit means being operative in response to said bistable multivibrator means remaining in said first state for a predetermined period of time to reset said bistable multivibrator means to said second state.

8. The detector of claim 7 wherein said predetermined period of time is greater than the period of one half cycle of the lowest particular tone frequency signals in said range of signals.

9. The detector of claim 8 wherein said switch means includes differential amplifier means having a first input, bias means coupled to said first input for coupling a bias voltage thereto equal to said integration voltage predetermined amplitude level, and second input means coupled to said integration means for receiving said integration voltage therefrom, said differential amplifier means being operative in response to said integration voltage reaching said predetermined amplitude level to develop said detection signal.

10. The detector of claim 9 wherein said bistable multivibrator means third circuit means includes first transistor means having base, said fourth circuit means includes second transistor means having base, emitter and collector electrodes, emitter and collector electrodes, said first and second transistor means being coupled together to form said bistable multivibrator means.

11. The detector of claim 10 wherein said reset means includes second integration means coupled to the bistable multivibrator means first transistor means and operative to develop a second integration voltage in response to said bistable multivibrator means remaining in said first state, and switching means coupled to said integration means and said bistable multivibrator means second transistor means and responsive to said second integration voltage exceeding a predetermined amplitude to switch said bistable multivibrator means to said second state.

12. The detector of claim 11 wherein said switch means further includes amplifier means coupled to said differential amplifier means, for amplifying said detection signal.

13. A detector for detecting any one of the particular tone frequency signals in a range of signals and having a constant detection time period, said detector including in combination; phase splitter means for receiving said particular tone frequency signals, said phase splitter means being operative to develop first tone fre quency signals and second tone frequency signals substantially 180 out of phase with said first tone frequency signals, multivibrator means being operable to first and second states, said multivibrator means including bias means coupling said phase splitter means to said multivibrator means, said bias means biasing said multivibrator means to switch states in response to said first and second tone frequency signals coupled thereto, said multivibrator means being responsive to said first and second tone frequency signals coupled thereto to switch between said first and second states and develop a square wave signal train having substantially a 50 percent duty cycle, integration means coupled to said multivibrator means and responsive to the square wave signals coupled thereto to develop an integration voltage, said integration voltage reaching a predetermined amplitude level in said constant duration time period, differential amplifier means having a first input, bias means coupled to said first input for coupling a bias voltage thereto equal to said integration voltage predetermined amplitude level, and second input means coupled to said integration means for receiving said integration voltage therefrom, said differential amplifier means being operative in response to said integration voltage reaching said predetermined amplitude level to develop said detection signal, and reset circuit means being operative in response to said multivibrator means remaining in said first state for a predetermined period of time to reset said multivibrator means to said second state. 

1. A detector for detecting any one of the particular sinusoidal tone frequency signals in a range of sinusoidal tone frequency signals and having a constant detection time period, said detector including in combination; converter means being operative in response to said particular sinusoidal tone frequency signals exceeding a predetermined threshold to develop square wave signaLs having substantially a 50 percent duty cycle, integration means coupled to said converter means and responsive to the square wave signals coupled thereto to develop an integration voltage, said integration voltage reaching a predetermined amplitude level in response to said square wave signals in said constant detection time period, and switch means coupled to said integration means and responsive to said integration voltage reaching said predetermined amplitude level to develop a detection signal.
 2. The detector of claim 1 wherein said converter means includes, phase splitter means for receiving said particular sinusoidal tone freuqency signals, said phase splitter means being operative to develop first tone frequency signals, and second tone frequency signals substantially 180* out of phase with said first tone frequency signals, multivibrator means having first and second states coupled to said phase splitter means and responsive to said first and second tone frequency signals exceeding a predetermined amplitude to switch between said first and second states and develop said square wave signals.
 3. A detector for detecting any one of the particular tone frequency signals in a range of signals and having a constant detection time period, said detector including in combination; first circuit means for receiving said particular tone frequency signals and having first and second outputs, said first circuit means being operative to develop said first tone frequency signals having a first phase at said first output and second tone frequency signals having a second phase at said second output, second circuit means having a first and second state coupled to said first and second output of said first circuit means, said second circuit means being responsive to said signals from said first and second outputs to switch between said first and second states and develop a square wave signal train having substantially a 50 percent duty cycle, integration means coupled to said second circuit means and responsive to the square wave signals coupled thereto to develop an integration voltage, said integration voltage reaching a predetermined amplitude level in said constant detection time period, and switch means coupled to said integration means and responsive to said integration voltage reaching said predetermined amplitude level to develop a detection signal.
 4. The detector of claim 3 wherein said first circuit means includes phase splitter means said phase splitter means being operative to develop said first and second tone frequency signals, said second tone frequency signals being substantially 180* out of phase with said first tone frequency signals, said first tone frequency signals having positive and negative half cycles of substantially equal period, said second circuit means including third circuit means responsive to one of said positive and negative half cycles coupled thereto from said first output to switch to said first state, and fourth circuit means responsive to one of said positive and negative half cycles coupled thereto from said second output to switch to said second state.
 5. The detector of claim 4 wherein said second circuit means includes bistable multivibrator means coupled to said phase splitter means first and second outputs, said bistable multivibrator means being responsive to said signals from said first output to switch to said first state and responsive to said signals from said second output to switch to said second state.
 6. The detector of claim 5 wherein said second circuit means further includes bias means coupling said phase splitter means first and second outputs to said bistable multivibrator means, third and fourth circuit means said bias means biasing said bistable multivibrator means to switch states in response to one of said positive and negative half cycles of said particular tone frequency signals coupled thereto.
 7. The detector of claim 6 wherein said second circuit means furTher includes reset circuit means being operative in response to said bistable multivibrator means remaining in said first state for a predetermined period of time to reset said bistable multivibrator means to said second state.
 8. The detector of claim 7 wherein said predetermined period of time is greater than the period of one half cycle of the lowest particular tone frequency signals in said range of signals.
 9. The detector of claim 8 wherein said switch means includes differential amplifier means having a first input, bias means coupled to said first input for coupling a bias voltage thereto equal to said integration voltage predetermined amplitude level, and second input means coupled to said integration means for receiving said integration voltage therefrom, said differential amplifier means being operative in response to said integration voltage reaching said predetermined amplitude level to develop said detection signal.
 10. The detector of claim 9 wherein said bistable multivibrator means third circuit means includes first transistor means having base, said fourth circuit means includes second transistor means having base, emitter and collector electrodes, emitter and collector electrodes, said first and second transistor means being coupled together to form said bistable multivibrator means.
 11. The detector of claim 10 wherein said reset means includes second integration means coupled to the bistable multivibrator means first transistor means and operative to develop a second integration voltage in response to said bistable multivibrator means remaining in said first state, and switching means coupled to said integration means and said bistable multivibrator means second transistor means and responsive to said second integration voltage exceeding a predetermined amplitude to switch said bistable multivibrator means to said second state.
 12. The detector of claim 11 wherein said switch means further includes amplifier means coupled to said differential amplifier means, for amplifying said detection signal.
 13. A detector for detecting any one of the particular tone frequency signals in a range of signals and having a constant detection time period, said detector including in combination; phase splitter means for receiving said particular tone frequency signals, said phase splitter means being operative to develop first tone frequency signals and second tone frequency signals substantially 180* out of phase with said first tone frequency signals, multivibrator means being operable to first and second states, said multivibrator means including bias means coupling said phase splitter means to said multivibrator means, said bias means biasing said multivibrator means to switch states in response to said first and second tone frequency signals coupled thereto, said multivibrator means being responsive to said first and second tone frequency signals coupled thereto to switch between said first and second states and develop a square wave signal train having substantially a 50 percent duty cycle, integration means coupled to said multivibrator means and responsive to the square wave signals coupled thereto to develop an integration voltage, said integration voltage reaching a predetermined amplitude level in said constant duration time period, differential amplifier means having a first input, bias means coupled to said first input for coupling a bias voltage thereto equal to said integration voltage predetermined amplitude level, and second input means coupled to said integration means for receiving said integration voltage therefrom, said differential amplifier means being operative in response to said integration voltage reaching said predetermined amplitude level to develop said detection signal, and reset circuit means being operative in response to said multivibrator means remaining in said first state for a predetermined period of time to reset said multivibrator means to said second state. 